This invention relates to a method and apparatus for filling cryogenic liquid cylinders. Typically a filling station will have a large storage tank, often referred to as stand tank, in which a fluid, such as oxygen, nitrogen, argon or carbon dioxide, is stored in liquid form. Portable cylinders, which are insulated to maintain the fluids in their liquid state, must be periodically refilled and transported to a place of use.
There exists a number of methods for filling cryogenic liquid cylinders from a stand tank. Most such methods incur filling losses in the range of 25-50% of what ultimately ends up in the cylinder. Because of this, the overall efficiency of such industrial gas distribution centers is typically between 80-95% which means that out of every 100 pounds delivered to the stand tank only 80-95 pounds gets sold to customers. Furthermore, to obtain even this efficiency requires that in some systems the filling operator be somewhat skilled and pay close attention to the filling operation.
A number of prior art systems have attempted to deal with these large filling losses. These systems include recirculating systems to prevent loss of flashed vapor, top filling the cylinder with pumps and pump aided pressure transfer systems. None of these is entirey satisfactory.
The recirculating system recirculates the flashed vapor generated when the liquid from the stand tank enters the liquid cylinder. Recirculating the flashed vapor back to the stand tank can result in a no loss system. However, there is a serious risk of contamination of the stand tank if a contaminated liquid cylinder is being filled. Further, a sophisticated operator is required because a pump is a necessary component of this system.
Top filling with a pump works only under ideal conditions wherein the plumbing between the stand tank and the liquid cylinder is precooled and the liquid cylinder is cold (that is, it has not been left empty for a period of days). Under typical conditions the cylinder must be blown down periodically to avoid losing pump prime or damaging the seals. Further, the operation takes ten to twelve minutes on average and requires a sophisticated operator to deal with the pump problems and maintenance.
Pump aided pressure transfer can be effective in reducing losses if the operator is skilled and takes the time to throttle the exhaust. However, this is difficult to optimize.
It is accordingly an object of the present invention to provide a method and apparatus for filling cryogenic liquid cylinders which significantly reduces filling losses.
A further object of the invention is to provide a low loss filling system which does not require a skilled operator and which is automatic in operation.
A further object of the invention is to provide a system which can attain over 95% filling efficiency without depending upon operator skill.
A further object of the invention is to maximize the transfer rate of cryogenic liquid while reducing or eliminating flash losses by minimizing the two phase flow that occurs when pressure of a saturated liquid is decreased.
Other objects and advantages of the invention will be apparent from the remaining portion of the specification.